org-html: add divs for pre/postamble defined in a new variable.

`org-export-html-divs' contains '("preamble" "content" "postamble")
and is used to define the <div "%s"> for this parts of the HTML file.
This commit also obsolete some variables that are not useful anymore
as the file time-stamp can (should) be inserted through the postamble
or the preamble instead.
Thanks to Sébastian Vauban for a preliminary version of this patch!
* org-html.el (org-export-html-with-timestamp)
(org-export-html-html-helper-timestamp): These are obsolete
variables as of Org version 7.7 as you can already export the
timestamp from the preamble or the postamble.
(org-export-html-before-content-div): Delete variable.
(org-export-html-content-div): obsolete variable as of 7.7.
(org-export-html-divs): New variable to define divs used in
HTML export.
(org-export-as-html): Now the preamble and the postamble are
surrounded by a <div ...>. The name of the div is defined
through `org-export-html-divs'.